<Regarding THP>
Regarding wireless communication technologies, Tomlinson Harashima precoding (THP) has been known. The THP is a technology in which under a condition that there is interference with communication between a transmission device and a reception device, the transmission device preliminarily detects the interference, preliminarily cancels the interference from a transmission signal, and transmits the resultant signal to the reception device. Here, the transmission device and the reception device transmits and receives a signal on which modulo arithmetic has been performed to suppress an increase in transmission power caused by cancelling the interference (see Non-Patent Document 1).
Hereinafter, communication using the THP is explained in detail.
Firstly, modulo arithmetic to be performed by the transmission device and the reception device is explained. This modulo arithmetic is arithmetic that adds an integral multiple of a value τ, which is known to the transmission device and the reception device, to an I-ch (In-phase channel) and a Q-ch (Quadrature channel) of a modulation symbol in order to convert that signal into a modulation symbol included within the range of [−τ/2, τ/2]. The modulo arithmetic is expressed by the following formula (1)
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      x          ′                =                                            Mod              τ                        ⁡                          (              x              )                                =                      x            -                                          floor                (                                                                            Re                      ⁡                                              (                        x                        )                                                              +                                          τ                      2                                                        τ                                )                            ⁢              τ                        -                                          j                ·                                  floor                  (                                                                                    Im                        ⁡                                                  (                          x                          )                                                                    +                                              τ                        2                                                              τ                                    )                                            ⁢              τ                                                          (        1        )            
Here, Modτ(x) denotes a modulo arithmetic. x denotes a modulation symbol to be subjected to the modulo arithmetic, and x′ denotes a modulation symbol resulting from the modulation arithmetic. j denotes an imaginary unit. Re(x) denotes a real part of x. Im(x) denotes an imaginary part of x. floor(x) denotes the maximum integer that does not exceed x.
FIG. 37 is a schematic diagram illustrating a modulo arithmetic according to related art. In this drawing, a modulation symbol appended with a reference symbol P11 denotes a modulation symbol to be subjected to the modulo arithmetic (x in the formula (1)). Additionally, a modulation symbol appended with a reference symbol P12 denotes a modulo symbol resulting from the modulo arithmetic (x′ in the formula (1)). Here, a modulation symbol P12 is a symbol obtained by adding (N1=−2, N2=−1 in the case of FIG. 37) to the modulation symbol P11.
In FIG. 37, the I-ch and the Q-ch of the modulation symbol P12 resulting from the modulo arithmetic are included within the range of [−τ/2, τ/2] from the origin. Thus, the amplitude of a signal can be included within a predetermined range by performing the modulo arithmetic, thereby enabling a decrease in transmission power.
Generally, in case that the average power for modulation symbols is normalized to 1, the modulo width τ becomes, according to a modulation scheme, a predetermined value preliminarily known to transmission and reception sides. For example, τ=2√{square root over (2)} for QPSK (Quadrature Phase Shift Keying), τ=8√{square root over (10)} for 16QAM (Quadrature Amplitude Modulation), or τ=16√{square root over (42)} for 64QAM.
Next, an interference cancelling using the modulo arithmetic is explained here. Here, a modulation symbol of a desired signal that the transmission device transmits to the reception device is denoted as a desired symbol s. A modulation symbol of interference between the transmission device and the reception device is denoted as an interference symbol f.
In the ease of communication using the THP, the transmission device subtracts the interference symbol f from the desired symbol s. Thus, the reception device can receive the desired symbol s by just demodulating a reception signal. However, the amplitude of an interference cancelled symbol s−f resulting from the subtraction is generally increased by subtracting the interference. For this reason, if the signal of this interference cancelled symbol is transmitted, the transmission power increases. Therefore, the transmission device performs the modulo arithmetic on this interference cancelled symbol s−f, and then transmits a signal of the modulo symbol x′ (=Modτ(s−f)) resulting from the arithmetic. Thereby, the transmission device can make the I-ch and the Q-ch of the modulation symbol of the signal to be transmitted be included within the range of [−τ/2, τ/2] from the origin. Thus, the transmission device can transmit a signal for which the power is suppressed compared to a case of transmitting the interference cancelled symbol s−f.
The signal transmitted by the transmission device is affected by interference, and the modulation symbol of that signal received by the reception device is a reception symbol y=Modτ(s−f)+f. Here, the channel characteristic is assumed to be 1, and thus the effect of noise is ignored. If the modulo arithmetic is performed on that reception symbol, then the result is Modτ{Modτ(s−f)+f}=Modτ(s−f+f)=Modτ(s)=s. In other words, the reception device can detect the desired symbol s.
The above is the mechanism for the communication using the THP.
<Regarding MU-MIMO THP>
Next, communication, in which the THP is used for multi-user MIMO (Multi-User Multi Input Multi Output) communication, is explained. Here, this communication technology for a downlink (DL) from a base station device to a terminal device is referred to as a DL MU-MIMO THP.
FIG. 38 is a schematic diagram illustrating a wireless communication system according to the related art. This drawing is a diagram illustrating a wireless communication system to which the DL MU-MIMO THP is applied.
In this drawing, a base station device X1 transmits signals to multiple terminal devices Y11 and Y12. If these signals are transmitted at the same time, using the same frequency, the signals interfere with each other (multi-user interference). The DL MU-MIMO THP is a technology of cancelling that multi-user interference.
Non-Patent Document 2 discloses the DL MU-MIMO THP.
Hereinafter, regarding the wireless communication system shown in FIG. 38, configurations of the base station device X1 and the terminal devices Y1 (Y11 and Y12) are explained.
FIG. 39 is a schematic block diagram illustrating the configuration of the base station device X1 according to the related art.
Regarding a multiplexed signal generator X13, a filter calculator X131 receives channel state information (CSI) from the terminal devices Y11 and Y12, and calculates an interference coefficient and a linear filter based on the received CSI. The filter calculator X131 outputs the calculated interference coefficient and the calculated linear filter to an interference calculator X132 and a linear filter multiplier X135, respectively.
The interference calculator X132 multiplies a modulation symbol s1 addressed to the terminal device Y11 received from a modulator X121, by the interference coefficient indicated by the information received from the filter calculator X131, to calculate an interference symbol f. The interference calculator X132 outputs the calculated interference symbol f to an interference subtractor X133.
The interference subtractor X133 subtracts the interference symbol f received from the interference calculator X132, from a modulation symbol s2 addressed to the terminal device Y12 received from a modulator X122. The interference subtractor X133 outputs to a modulo arithmetic unit X134, the interference cancelled symbol s2−f resulting from the subtraction.
The modulo arithmetic unit X134 performs the modulo arithmetic shown in the formula (1) on the interference cancelled symbol received from the interference subtractor X133. Then, the modulo arithmetic unit X134 outputs to a linear filter multiplier X135, a modulo symbol s2′ (=Modτ(s2−f)) resulting from the arithmetic.
The linear filter multiplier X135 (coefficient multiplier) multiplies, by the linear filter indicated by the information received from the filter calculator X131, the modulation symbol s1 addressed to the terminal device Y11 and received from the modulator X121, and the modulo symbol s2′ received from the modulo arithmetic unit X134. Then, the linear filter multiplier X135 outputs the results to the wireless transmitters X141 and X142.
By the above process, the base station device X1 can make an element of the signal addressed to the terminal device Y12 be 0 (null) with respect to the direction from the base station device X1 to the terminal device Y11. The principle of this operation will be explained later.
FIG. 40 is a schematic block diagram illustrating a configuration of the terminal device Y1 according to the related art. In this drawing, a modulo arithmetic unit Y113 performs the modulo arithmetic shown in the formula (1) on a modulation symbol of a reception signal having been subjected to channel compensation, and thereby extracts a desired symbol.
The principle of operation regarding a wireless communication system, to which the DL MU-MIMO THP shown in FIGS. 38 to 40 is applied, is explained here.
Regarding the base station device X1, an interference calculator X132 performs QR decomposition on an Hermitian conjugate HH of a channel matrix H. The QR decomposition is to decompose a matrix into a unitary matrix Q and an upper triangular matrix R. This decomposition can be expressed by the following formula (2).
                    [                  Formula          ⁢                                          ⁢          2                ]                                                                      H          H                =                  QR          =                      Q            ⁡                          (                                                                                          r                      11                                                                                                  r                      12                                                                                                            0                                                                              r                      22                                                                                  )                                                          (        2        )            
The filter calculator X131 generates the matrix HH using the CSI, and performs QR decomposition on the matrix HH. The filter calculator X131 calculates the matrix Q as a linear filter, and calculates r12*/r22* as an interference coefficient. Here, r* denotes a complex conjugate of r.
The interference calculator X132 calculates the interference symbol f as (r12*/r22*)s1. Additionally, the modulo arithmetic unit X134 generates the modulo symbol s2′ as Modτ{s2−(r12*/r22*)s1}, and outputs the generated symbol s2′ to the linear filter multiplier X135. The linear filter multiplier X135 generates symbols s1″ and s2″ shown in the following formula (3), and outputs the generated symbols to the wireless transmitters X141 and X142, respectively.
                    [                  Formula          ⁢                                          ⁢          3                ]                                                                      (                                                                      s                  1                  ″                                                                                                      s                  2                  ″                                                              )                =                  Q          ⁡                      (                                                                                s                    1                                                                                                                    s                    2                    ′                                                                        )                                              (        3        )            
A reception signal received by the terminal device Y11 is denoted as a reception symbol y1. A reception signal received by the terminal device Y12 is denoted as a reception symbol y2. y1 and y2 are expressed by the following formula (4).
                    [                  Formula          ⁢                                          ⁢          4                ]                                                                                                                (                                                                                                    y                        1                                                                                                                                                y                        2                                                                                            )                            =                            ⁢                                                H                  ⁡                                      (                                                                                                                        s                            1                            ″                                                                                                                                                                            s                            2                            ″                                                                                                                )                                                  =                                                                                                    (                        QR                        )                                            H                                        ⁢                                          Q                      ⁡                                              (                                                                                                                                            s                                1                                                                                                                                                                                                        s                                2                                ′                                                                                                                                    )                                                                              =                                                                                    (                                                                              R                            H                                                    ⁢                                                      Q                            H                                                                          )                                            ⁢                                              Q                        ⁡                                                  (                                                                                                                                                      s                                  1                                                                                                                                                                                                                      s                                  2                                  ′                                                                                                                                              )                                                                                      =                                                                  R                        H                                            ⁡                                              (                                                                                                                                            s                                1                                                                                                                                                                                                        s                                2                                ′                                                                                                                                    )                                                                                                                                                                    =                            ⁢                              (                                                                                                                              r                          11                          *                                                ⁢                                                  s                          1                                                                                                                                                                                                                              r                            12                            *                                                    ⁢                                                      s                            1                                                                          +                                                                              r                            22                            *                                                    ×                                                      Mod                            ⁡                                                          (                                                                                                s                                  2                                                                -                                                                                                      (                                                                                                                  r                                        12                                        *                                                                            /                                                                              r                                        22                                        *                                                                                                              )                                                                    ⁢                                                                      s                                    1                                                                                                                              )                                                                                                                                                                          )                                                                        (        4        )            
The formula (4) shows that the reception symbol y1 (=r11*s1) received by the terminal device Y11 does not include an element of the desired symbol s2 addressed to the terminal device Y12. In other words, the base station device X1 can make an element of the signal addressed to the terminal device Y12 be 0 (null) with respect to the direction from the base station device X1 to the terminal device Y11.
Regarding the terminal device Y11, a channel compensator Y112 divides the reception symbol y1 by r11*, and thereby can extract the desired symbol s1. Here, the modulo arithmetic unit Y113 performs the modulo arithmetic. Since Modτ(s1)=s1, the desired symbol s1 is output to the demodulator Y114.
Regarding the terminal device Y12, the channel compensator Y112 divides the reception symbol y2 by r22*. The channel compensator Y112 outputs to the modulo arithmetic unit Y113, a channel compensated symbol z2(=y2/r22*) resulting from the division.
The modulo arithmetic unit Y113 performs the modulo arithmetic shown in the formula (1) on the channel compensated symbol z2 to extract the desired symbol s2 (see the following formula (5)).
                    [                  Formula          ⁢                                          ⁢          5                ]                                                                                                                                  Mod                  τ                                ⁡                                  (                                      z                    2                                    )                                            =                            ⁢                                                Mod                  τ                                ⁡                                  (                                                                                                              r                          12                          *                                                /                                                  r                          22                          *                                                                    ⁢                                              s                        1                                                              +                                                                  Mod                        τ                                            ⁡                                              (                                                                              s                            2                                                    -                                                                                                                    r                                12                                *                                                            /                                                              r                                22                                *                                                                                      ⁢                                                          s                              1                                                                                                      )                                                                              )                                                                                                        =                            ⁢                                                Mod                  τ                                ⁡                                  (                                                                                                              r                          12                          *                                                /                                                  r                          22                          *                                                                    ⁢                                              s                        1                                                              +                                          (                                                                        s                          2                                                -                                                                                                            r                              12                              *                                                        /                                                          r                              22                              *                                                                                ⁢                                                      s                            1                                                                                              )                                                        )                                                                                                        =                            ⁢                              s                2                                                                        (        5        )            
Additionally, Non-Patent Document 3 discloses a wireless communication system to which the aforementioned DL MU-MIMO THP is applied, in a case where each terminal device includes multiple antennas and performs SU-MIMO (Single-User Multi Input Multi Output).
FIG. 41 is another schematic diagram illustrating a wireless communication system according to the related art. This drawing illustrates a wireless communication system to which the DL MU-MIMO THP is applied, in a case where each terminal device performs SU-MIMO communication.
In this drawing, a base station device X2 transmits, using the SU-MIMO communication, signals to multiple terminal devices Y21 to Y23. If those signals are transmitted at the same time, using the same frequency, those signals cause multi-user interference with each another. However, this multi-user interference is cancelled by applying the DL MU-MIMO THP.